Is there a way to find all vertices mapped to a specific UV image
A slight variation of this answer, if you paste the following script into blenders python console you will get a list of what face uses which material and image.

import bpy

for obj in bpy.context.scene.objects:
    if obj.type == 'MESH':
        for f in obj.data.polygons:
            mat = obj.material_slots[f.material_index].material
            if mat and mat.use_nodes:
                for n in mat.node_tree.nodes:
                    if n.type == 'TEX_IMAGE':
                        print('Face', f.index, 'uses', n.image.name, 'from', mat.name)


If you are using 2.80, while in edit mode enable indices in the overlays menu.



Now in face select mode, when you select a face you will see the face index. Match this with the face index from the script to see what image it uses.

If you are using 2.7x, first enter bpy.app.debug=True in the python console, then you will find the indices option under Mesh Display, everything else is the same.